IPv6 Router Advertisement Guard

Abstract

Routed protocols are often susceptible to spoof attacks. The
canonical solution for IPv6 is Secure Neighbor Discovery (SEND), a
solution that is non-trivial to deploy. This document proposes a
light-weight alternative and complement to SEND based on filtering in
the layer-2 network fabric, using a variety of filtering criteria,
including, for example, SEND status.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6105.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

1. Introduction

When operating IPv6 in a shared layer-2 (L2) network segment without
complete SEcure Neighbor Discovery (SEND) support by all devices
connected or without the availability of the infrastructure necessary
to support SEND [RFC3971], there is always the risk of facing
operational problems due to rogue Router Advertisements (RAs)
generated maliciously or unintentionally by unauthorized or
improperly configured routers connecting to the segment.

There are several examples of work done on this topic that resulted
in related studies and code, including [NDPMON] [KAME]
[IPv6-SAMURAIS]. This document describes a solution framework for
the rogue-RA problem [RFC6104] where network segments are designed
around a single L2-switching device or a set of L2-switching devices
capable of identifying invalid RAs and blocking them. The solutions
developed within this framework can span the spectrum from basic
(where the port of the L2 device is statically instructed to forward
or not to forward RAs received from the connected device) to advanced
(where a criterion is used by the L2 device to dynamically validate
or invalidate a received RA, this criterion can even be based on SEND
mechanisms).

2. Model and Applicability

RA-Guard applies to an environment where all messages between IPv6
end-devices traverse the controlled L2 networking devices. It does
not apply to shared media, when devices can communicate directly
without going through an RA-Guard-capable L2 networking device.

Figure 1

RA-Guard does not intend to provide a substitute for SEND-based
solutions. It actually intends to provide complementary solutions in
those environments where SEND might not be suitable or fully
supported by all devices involved. It may take time until SEND is
ubiquitous in IPv6 networks and some of its large-scale deployment
aspects are sorted out, such as provisioning hosts with trust
anchors. It is also reasonable to expect that some devices, such as
IPv6-enabled sensors, might not consider implementing SEND at all.
An RA-Guard implementation that SEND-validates RAs on behalf of hosts
would potentially simplify some of these challenges.

RA-Guard can be seen as a superset of SEND with regard to router
authorization. Its purpose is to filter Router Advertisements based
on a set of criteria, from a simplistic "RA disallowed on a given
interface" to "RA allowed from pre-defined sources" and up to a full-
fledged SEND "RA allowed from authorized sources only".

In addition to this granularity on the criteria for filtering out
Router Advertisements, RA-Guard introduces the concept of router
authorization proxy. Instead of each node on the link analyzing RAs
and making an individual decision, a legitimate "node-in-the-middle"
performs the analysis on behalf of all other nodes on the link. The
analysis itself is not different from what each node would do: if
SEND is enabled, the RA is checked against X.509 certificates

[RFC4861]. If any other criterion is in use, such as known L3
(addresses) or L2 (link-layer address, port number) legitimate
sources of RAs, the node-in-the middle can use this criterion and
filter out any RA that does not comply. If this node-in-the-middle
is an L2 device, it will not change the content of the validated RA
and will avoid any of the ND-proxy pitfalls.

RA-Guard intends to provide simple solutions to the rogue-RA problem
in contexts where simplicity is required while leveraging SEND in a
context environment consisting of a mix of SEND-capable devices (L2
switches and routers) and devices that do not consistently use SEND.
Furthermore, RA-Guard is useful to simplify SEND deployments, as only
the L2 switch and the routers are required to carry certificates
(their own and the trust anchor certificates).

3. Stateless RA-Guard

Stateless RA-Guard examines incoming RAs and decides whether to
forward or block them based solely on information found in the
message or in the L2-device configuration. Typical information
available in the frames received, useful for RA validation, is as
follows:

Link-layer address of the sender

Port on which the frame was received

IP source address

Prefix list

The following configuration information created on the L2 device can
be made available to RA-Guard, to validate against the information
found in the received RA frame:

Allowed/Disallowed link-layer address of the RA sender

Allowed/Disallowed ports for receiving RAs

Allowed/Disallowed IP source addresses of the RA sender

Allowed Prefix list and Prefix ranges

Router Priority

Once the L2 device has validated the content of the RA frame against
the configuration, it forwards the RA to its destination, whether
unicast or multicast. Otherwise, the RA is dropped.

An example of a very simple stateless RA-Guard implementation could
be a small L2 switch for which there is one interface "statically
configured" as the interface connecting to a router, while all other
interfaces are for non-router devices. With this small static setup,
the only interface forwarding RAs will be the pre-assigned router
interface, while the non-router interfaces block all RAs.

4. Stateful RA-Guard

4.1. State Machine

Stateful RA-Guard learns dynamically about legitimate RA senders and
stores this information for allowing subsequent RAs. A simple
stateful scheme would be for the L2 device to listen to RAs during a
certain manually configured period of time, where the start of the
listening period and the duration of the listening period for a
single instance are controlled by the manual intervention. As a
result, the L2 device can then allow subsequent RAs only on those
ports on which valid RAs were received during this period. Often,
the "LEARNING" state will only be activated by manual configuration
when a new IPv6 router is provisioned on the L2 network.

A more sophisticated stateful scheme is based on SEND and is
described in Section 4.2.

The state machine for stateful RA-Guard can be global, per-interface,
or per-peer, depending on the scheme used for authorizing RAs.

When RA-Guard is SEND-based, the state machine is per-peer and
defined in [RFC3971].

When RA-Guard is using a discovery method, the state machine of the
RA-Guard capability consists of four different states:

State 1: OFF

A device or interface in the RA-Guard "OFF" state operates as if
the RA-Guard capability is not available.

State 2: LEARNING

A device or interface in the RA-Guard "LEARNING" state is actively
acquiring information about the IPv6 routing devices connected to
its interfaces. The learning process takes place over a
pre-defined unique period of time, as set by manual configuration;
or it can be event-triggered. The information gathered is
compared against pre-defined criteria (criteria similar to the
stateless RA-Guard rules) to qualify the validity of the RAs.

In this state, the RA-Guard-enabled device or interface is either
blocking "all" RAs until their validity is verified or,
alternatively, it can temporarily forward "all" of the RAs until
their validity is verified.

When the L2 device reaches the end of the LEARNING state, it has a
record of which interfaces are attached to links with valid IPv6
routers. The L2 device transitions each interface from the
LEARNING state into either the BLOCKING state if there was no
valid IPv6 router discovered at the interface, or into the
FORWARDING state if there was a valid IPv6 router discovered.

State 3: BLOCKING

A device or interface running RA-Guard and in the BLOCKING state
will block ingress RA messages.

An interface can transition from the BLOCKING state into the
FORWARDING state directly if explicitly instructed by the
L2-device operator.

An interface can transition from the BLOCKING state into the
LEARNING state if either explicitly instructed by the L2-device
operator or prompted by a triggered event.

State 4: FORWARDING

A device or interface running RA-Guard and in the FORWARDING state
will accept valid ingress RAs and forward them to their
destination.

An interface can transition from the FORWARDING state into the
BLOCKING state directly if explicitly instructed by the L2-device
operator.

An interface can transition from the FORWARDING state into the
LEARNING state if either explicitly instructed by the L2-device
operator or prompted by a triggered event.

The transition between these states can be triggered by manual
configuration or by meeting a pre-defined criterion.

4.2. SEND-Based RA-Guard

In this scenario, the L2 device is blocking or forwarding RAs based
on SEND considerations. Upon capturing an RA on the interface, the
L2 device will first verify the Cryptographically Generated Address
(CGA) [RFC3971] and the RSA (Rivest, Shamir, and Adleman algorithm
for public-key cryptography) signature, as specified in Section 5 of
[RFC3971]. The RA should be dropped in case of failure of this
verification. It will then apply host behavior as described in
Section 6.4.6 of [RFC3971]. In particular, the L2 device will
attempt to retrieve a valid certificate from its cache for the public
key referred to in the RA. If such a certificate is found, the L2
device will forward the RA to its destination. If not, the L2 device
will generate a Certification Path Solicitation (CPS) [RFC3971] with
an unspecified source address, to query the router certificate(s).
It will then capture the Certification Path Advertisement (CPA)
[RFC3971] and attempt to validate the certificate chain. Failure to
validate the chain will result in dropping the RA. Upon validation
success, the L2 device will forward the RA to its destination and
store the router certificate in its cache.

In order to operate in this scenario, the L2 device should be
provisioned with a trust anchor certificate, as specified in
Section 6 of [RFC3971]. It may also establish layer-3 connectivity
with a Certificate Revocation List (CRL) Certification Path
Advertisement server and/or with an NTP server. A bootstrapping
issue in this case can be resolved by using the configuration method
to specify a trusted port to a first router, and the SEND-based
RA-Guard method on all other ports. The first router can then be
used for Network Time Protocol (NTP) [RFC5905] and CRL connectivity.

5. RA-Guard Use Considerations

The RA-Guard mechanism is effective only when all messages between
IPv6 devices in the target environment traverse controlled L2
networking devices. In the case of environments such as Ethernet
hubs, devices can communicate directly without going through an
RA-Guard-capable L2 networking device, and the RA-Guard feature
cannot protect against rogue RAs.

RA-Guard mechanisms do not offer protection in environments where
IPv6 traffic is tunneled.

6. Security Considerations

Once RA-Guard has set up the proper criteria (for example, it
specified that a port is allowed to receive RAs, or it identified
legitimate sources of RAs or certificate bases [RFC4861]), then there
are no possible instances of accidentally filtered legitimate Router
Advertisements, assuming the RA-Guard filter enforcement strictly
follows the RA-Guard set criteria.

In Section 4.1, a simple mechanism to dynamically learn the valid
IPv6 routers connected to an L2 device is explained. It is important
that this LEARNING state is only entered intentionally by manual
configuration. The list of learned IPv6 routers should be verified
by the network manager to make sure that it corresponds with the
expected valid RA list. This procedure will make sure that either
accidentally or intentionally generated rogue RAs are blocked by
RA-Guard.

7. Acknowledgements

The authors dedicate this document to the memory of Jun-ichiro Hagino
(itojun) for his contributions to the development and deployment of
IPv6.